1. The Present Invention
The present invention concerns a device to support washpipe in a downhole situation and, in particular to a device which will maintain concentricity during cementing operations.
2. Prior Art
When performing subsea wellhead tiebacks or mudline suspension wellhead tiebacks, it is required that certain pipe strings be reconnected to the surface in order to prepare the well for its completion. These operations require some smaller diameter pipe strings to be run through the inside of some larger diameter pipe strings. During this procedure, as well as in other special cases, it may be desirable to form an additional pressure seal by the use of a cement column between these pipe strings. This is called "grouting" the pipe annulus.
Centralization is preferred when running the smaller diameter pipe strings through the larger diameter strings to prevent the buckling of the smaller inner strings. This is accomplished by installing metal centralizers on the inner string. This helps to keep the smaller string centered inside the larger diameter pipe, by restricting lateral movement, thereby reducing the chance of buckling. In addition to reducing the buckling tendency, centralization also acts to increase the chances of obtaining a good cement bond during grouting since it offers all areas of the annulus equal opportunity for cement coverage.
The conventional method of performing a grout between casing strings calls for the running of a small pipe string (called washpipe) down the annulus formed by the casing and inner pipestring. The washpipe is normally on the order of one quarter to one eighth the diameter of the outer casing. When conditions dictate grouting large areas, it may be difficult to place cement on all sides of the annulus using a single washpipe string. The solution, of course, would be to use plural strings of washpipe. This in itself presents a problem in that unless the several washpipe strings are separated, they may all wind up on the same side of the annulus when the cement is pumped thereby decreasing the chance of a proper cement seal. To prevent this problem, centralizers may be used to restrain the relative movement of the washstrings as well as lateral movement of the inner pipe string. The major problem is that when centralization is used for the inner pipe string, it is often difficult to run even a single string of washpipe in the annulus since the washstring, in travelling downhole, may hang up on the centralizer arms.
One solution to running the washstring to the desired depth, without having to worry about its passage past the centralizers, is to simultaneously run one or more washstring(s) attached to the smaller diameter inner pipe. In other words, the grout string and the smaller diameter inner casing string are run at the same time through the larger diameter outer casing string. In the past this has been attempted using metal bands to hold the washstring onto the inner casing string. While the inner casing string can be supported by tools at the rig floor, the washpipes have no means for supporting their own weight other than through these metal bands holding them to the inner casing string. For this reason, the metal bands must be installed relatively tightly around the washstring and the inner casing string. The problem using such metal bands occurs after the cement has been pumped and the washpipe string is being retrieved to the surface. Often these metal bands will slip to accumulate in one area as the washpipe is being pulled from the annulus. At some point during the retrieval of washpipe, the accumulation of the metal bands may be such that they bind the washpipe and prevents it from being pulled completely out of the annulus. This could result in the washpipe being cemented in the hole and the operator having to purchase this normally rented string of pipe.